Impact printers of the type with which the present invention is primarily concerned effect printing "on-the-fly" at relatively high speeds. In such printers the character dies or type are mounted in a longitudinal array on a continuously moving carrier. The carrier is drawn past an aligned array of selectively actuable print hammers. Interposed between the character dies and the print hammers is a print medium in the form of a paper sheet and a reversibly driven ink or carbon impregnated ribbon. The type characters are struck when they are in motion with the energy for impacting delivered by an inertial hammer mechanism. An inertial hammer mechanism is one wherein the hammer is thrown much in the manner of a projectile by a hammer operator such as an electromagnet. The hammers are individually subjected to an abrupt impact force and propelled against the paper under their own momentum.
In order to maximize printing speed, the hammers should have a low mass and be propelled at high speed with minimal friction. While such hammer operation is conducive to high speed printing, it has heretofore imposed a serious problem in regard to effectively damping the kinetic energy of the hammer return force, so as to reduce hammer rebound. Such rebound forces are often augmented by a hammer return spring. If these return forces are not absorbed or damped, they can very readily cause a given print hammer, after reaching a backstop, to rebound off the backstop in a forward direction with sufficient velocity to again strike the paper producing undesired character images resulting in blurred print or ghosting.
Several approaches have been suggested for reducing the rebound motion of the hammers. One class of printers uses the magnetic attractive force of core pole faces of an energized solenoid to effect the "firing" of an associated hammer. A predetermined amount of current is maintained in the solenoid coil during the rebound return of the hammer thus producing a dynamic braking action against the returning hammer. A particular disadvantage with such an arrangement is that the duty cycle of the solenoid is greatly increased as is the power consumption of the system.
In other systems, the armature of the solenoid is used directly or indirectly, through a coupled actuator, to "fire" the associated hammer. A second electromagnet has been employed to magnetically attract the rebound-returned hammer against an associated backstop from which the hammer is again fired. With such an arrangement the hammer mechanism is relatively complex increasing cost and further necessitating a substantial increase in the current requirements of the print hammer drive circuitry.
A simplified approach utilizes a resilient shock absorbing bumper positioned to absorb the rebound forces of the hammer. Such a structure is described in U.S. Pat. No. 3,994,218 issued Nov. 30, 1976 to Arthur F. Riley and entitled "Energy Absorbing Print Hammer Bumper with Internal Stabilizer." A similar resilient bumper is described in U.S. Pat. No. 3,823,667 issued July 16, 1974 to E. S. Babler entitled "Force Adjustment in Impact Printers." It will be appreciated that the material used for such a rebound absorbing bumper must absorb a great portion of the rebound energy; that is, it should possess a relatively low resiliency.
On-the-fly printers are frequently used to print through multiple paper sheets interleaved with carbon paper. Usually, means are provided permitting the operator to manually adjust the hammer force to correspond with the number of layers being printed. If excessive force is applied to a single sheet, the paper and/or ink ribbon may be cut and conversely insufficient force applied to a multiple ply copy will produce faint print on the copy. An automatic means for adjusting the forward hammer pressure applied to the paper is described by M. E. Bear and J. E. McGuire in an IBM Technical Disclosure Bulletin entitled "Print Hammer Impact Control Mechanism" (Vol. 5, No. 11, April 1963). As described in this publication, a fixed stop carrying a pad is positioned to absorb the forward impact of the hammer with the amount of energy absorbed being related to the thickness of the print medium. When multiple paper thicknesses are placed in the printer, the hammer strikes the paper on the print stroke before or quickly after reaching the pad and thus the hammer impacts the paper with substantially maximum force. However, when a single ply sheet is used, the hammer, on the forward stroke, hits the pad well before striking the paper and a relatively large portion of the impact energy is absorbed by the pad during its compression thus preventing the application of excessive force to the paper.
A similar print force adjusting arrangement is described in U.S. Pat. No. 3,144,821 issued to J. E. Drejza on Aug. 18, 1964 and entitled "Printer Apparatus Having Print Force Control." Both the Drejza and Bear references disclose apparatus wherein the compressible pad is positioned out of the direct path of the hammer return stroke since the resiliency of the pad is high and therefore would not adequately dampen the hammer energy on the return stroke. Thus, heretofore the dampening of the return stroke of the print hammer and the compensation or adjustment of the hammer forward stroke have frequently been viewed as independent problems necessitating separate solutions. A bumper fabricated of a single material as in the aforecited U.S. Pat. No. 3,823,667 will not adequately accomplish both objectives due to the different resiliency and hardness requirements which the two operations require. The illustrated embodiment of this invention provides a novel solution to both of these problems by utilizing a unitary bumper assembly for a printer which absorbs the hammer rebound energy as well as automatically adjusts the hammer printing force to the number of copy plys being printed. A particular feature of the apparatus is that both of these advantages are provided by a relatively simple unitary bumper which is relatively economical to fabricate and utilize.